Shinji Touasaikuhara, orphaned by a childhood accident, grew up in an institution. Years later, the maid Zero informs him his deceased father was heir to a wealthy millionaire grandfather. This grandfather demands Shinji inherit the family business, but Shinji refuses due to his lack of connection and existing life plans—specifically his mutual marriage promise to his childhood friend Sae.

To escape inheritance, Shinji discovers an opt-out clause requiring him to find a girl bearing a cherry blossom-shaped birthmark on the back of her neck, which manifests only after sexual climax. Though committed to Sae, he agrees to the search as his sole path to freedom. Zero, representing his grandfather's interests, provides female candidates. Shinji pursues intimate encounters with them to reveal the birthmark, struggling to maintain loyalty to Sae throughout.

His dynamic with Zero remains professionally complex as she facilitates his quest. Shinji's core motivation is resolving the search swiftly to reclaim his planned future with Sae. The outcome of his efforts and its effect on his relationship remains unresolved in the original media.